Background
I Recently updated my OP5 machine later after an reboot got the error message. Important to know is that i run my VM (Virtual Machine) on a headless server so i have no more tools then to VNC to a VM and therefore its hard to get into grub at boot as the timeout was set to 5 seconds.
Kernel panic - not syncing: VFS: Unable to mount root fs on unit block(0,0)
information about diskfile
First we need to know what kind of fileformat is used to store our VM machine
[root@hem-dalesjo-net OP5]# qemu-img info /var/lib/libvirt/images/OP5.dsk image: /var/lib/libvirt/images/OP5.dsk file format: qcow2 virtual size: 48G (51539607552 bytes) disk size: 12G cluster_size: 65536 Format specific information: compat: 1.1 lazy refcounts: true
You can also do a quick check if the file is corrupt or not
[root@hem-dalesjo-net OP5]# qemu-img check /var/lib/libvirt/images/OP5.dsk No errors were found on the image. 786432/786432 = 100.00% allocated, 0.00% fragmented, 0.00% compressed clusters Image end offset: 51547734016
Install guestmount
These tools are needed to mount qcow2 files
yum install libguestfs libguestfs-bash-completion.noarch
Mount qcow2
virsh destroy OP5 mkdir /tmp/OP5/ /tmp/OP5/sda1 /tmp/OP5/lv_root/ guestmount -a /var/lib/libvirt/images/OP5.dsk -m /dev/sda1 /tmp/OP5/sda1/ guestmount -a /var/lib/libvirt/images/OP5.dsk -m /dev/vg_op5/lv_root /tmp/OP5/lv_root/
Edit grub
changed timeout to 35 seconds, AKA timeout=35
nano /tmp/OP5/sda1/grub/grub.conf
unmount qcow2
guestunmount /tmp/OP5/sda1/ && guestunmount /tmp/OP5/lv_root/ virsh start OP5
Solution
OP5 was recently updated to Kernel 2.6.32-696.18.7.el6.x86_64 which apparently is not bootable. changed default=1 in grub.conf to boot on the older kernel 2.6.32-696.16.1.el6.x86_64